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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
used to fill the occluded areas. This is illustrated in Figure5. 
Both points A, B have the gray value of pint B. 
Perspective 
Center 
Figure5: Double mapping. 
To restore the occluded areas, it is necessary to detect them and 
use neighboring overlap images to extract the information of 
these areas for replacing them in the orthophoto. 
• Distance to occluded areas: smaller distance to 
occluded areas will result in a disharmonious image. 
• View angle: narrowest view angle will avoid 
smearing problems. 
When the images are mosaicked the differences between 
radiometric characteristics of each image patch make seam lines 
visible. Consequently, the histogram matching is inevitable to 
equalize the radiometric responses of the image patches in order 
to achieve a true orthophoto with high quality. 
Perspective 
Center 
In the aerial Photogrammetry the relief displacements can be 
decreased by using images created with a normal angle lens shot 
from a higher altitude, but in the close rang photogrammetry 
this condition cannot be easily fulfilled and hence large relief 
displacements occur. So the occluded areas detection will be 
time consuming and an efficient method must be used. 
Figure 6: The principle of the height-based raytracing method 
for occluded area detection. 
4 EXPERIMENTAL RESULTS 
3.1 Occluded areas detection 
The detection of occluded areas is based on the visibility 
analysis. If a ray from the output pixel intersects other features 
in its path from DSM to perspective center, it is marked an 
occluded pixel and filled with black color. 
In this paper the height-based raytracing method is used for 
detecting occluded areas (Bang, 2007). In this method, for each 
pixel, the projection of ray onto the orthophoto plane (from the 
nadir point to the pixel in the orthophoto space) is considered as 
a search path. For testing the visibility of each pixel, the height 
of the ray is compared with the height of some points along the 
search path in specific intervals. If at least one point is found 
with higher height than the ray height, the pixel will be marked 
as occlusion. Figure 6 shows the principle of the height-based 
raytracing method. 
3.2 Mosaicking 
In this stage the information of the occluded areas are restored 
using the neighboring overlap images. In producing the true 
orthophoto each occluded pixel in image have to be restored 
with pixels in neighboring overlap images. So we must know 
which pixel in which image must be used. This selection is 
based on one or a combination of the following factors 
(Schickler, 1998): 
• Distance to nadir: smaller distance to nadir will result 
in a smaller relief displacement for features not 
modeled. 
In this paper four close range images from four different 
stations around the test object are used. The camera positions 
are selected in such a way that by combining the images no 
occluded areas remain. The positions of the stations are shown 
in figure7. 
Some retro-reflective targets are used on the test object to 
determine the position and orientation of the camera. The DLT 
equations are used as a camera model.